1. Field
The invention relates to an optical component comprising a layer structure made up of an anisotropic layer of cross-linked liquid crystalline monomers in contact with an orientation layer on a substrate. The invention also relates to the production and preferred use of the aforementioned components.
2. Description
Anisotropic transparent or colored polymer network layers with the optical axis oriented in three dimensions, either uniformly or pre-set point by point, are of great importance in many branches of display technology, integrated optics, etc.
Substances having these properties in principle, that is certain cross-linkable liquid crystalline diacrylates and diepoxides, have been known for some years. These substances in monomer form (before cross-linking) can be oriented in cells by means of conventional orientation layers or under the action of external fields (such as magnetic or electric fields) in the LC phase and can be photo cross-linked in a second stage by conventional means (irradiation with non-polarized light) without losing the orientation impressed in the monomeric state.
Layer structures of this kind are known, see EP-A-331 233. (Throughout this specification documents are identified. The contents of all of these documents are herein incorporated by reference). They are produced by orienting a monomer layer in an external field and then irradiating part of it through a mask. Cross-linking is initiated only in the irradiated region. The direction of the external field is then changed, and the monomer regions which have not yet been cross-linked are re-oriented in the new field direction. The latter region is then cross-linked by illumination. Clearly, this method cannot result in an orientation structure with high local resolution, since, owing to the shading of the mask, the radical cross-linking reaction is not sharply defined.
Methods of generating orientation layers with locally variable orientation properties have become known recently. For example, U.S. Pat. No. 4,974,941 describes orientation by photolithography of dichroic dye molecules incorporated in the polymer.
Layer structures comprising a film of cross-linked liquid crystalline monomers in contact with an orientation layer of a photo-orientatable polymer network (PPN) are described in European Patent Publication No. 0 611 981, published Aug. 24, 1994. These layer structures are produced by orienting the liquid crystalline monomers by interaction with the PPN layer and by fixing the orientations in a subsequent cross-linking step. For other details, reference should be made to concurrently filed U.S. patent application Ser. No. 08/489,865 now abandoned in favor of 08/721,509, the contents of which are herein incorporated by reference, which corresponds to Swiss Patent Application No. 2016/94.
Owing to the high-quality copying techniques which are available, there is an increasing need and requirement to prevent forgery of security elements in banknotes, credit cards, documents, etc., which however must clearly be recognized for what they are by the general public. Most security features in use at present, for example holograms or kinegrams (J. F. Moser, "Document Protection by Optically Variable Graphics", in "Optical Document Security", R. L. van Renesse (ed.), Artech House, Boston, London, page 169) are based on diffraction of light on gratings. To check a document's authenticity, these safety features must be observed at different angles of vision. This results in three-dimensional images, iridescence, or cinematic effects which have to be examined in accordance with rules specific to the pattern. This results in the difficulty that the general public needs to know a number of these complicated rules in order visually to check that a security element of this kind is genuine. Security features based on diffraction are also very complicated to check, particularly mechanically. There is no practical possibility of manual reading-in of individual items, for example images or numbers coded by these techniques.
EP-A 435 029 describes security elements in the form of cholesteric liquid crystals, of the kind known for example in liquid crystal thermometers. Cholesteric liquid crystals, like mechanical gratings, have a periodic structure. This property, known since 1888, results in the color of reflected light varying with the angle of observation. Unfortunately, the test for genuineness based on the associated iridescence is beset with the same problems as with mechanical gratings.
The invention provides other possibilities for optical and electro-optical components and equipment, using layer structures of the above-mentioned kind.